Brannigan, Eric M. | Search NASA

DOE OSTI2018

Tip radius variation with elastic indentation depth

The reduced elastic modulus of a material is measured with a nanoindenter probe that is operated in the tapping mode. The resonant frequency of a freely oscillating cantilever is reduced when contact is made between the indenter tip and surface under investigation. It’s shown using elasticity theory that the elastic deformation is a function of the indenter tip radius. A deeper penetration within the elastic range can change the tip radius, and introduce an error of 10% in calculating the reduced elastic modulus.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Strain-rate sensitivity of strength dependence on the softening factor c b in Ti-alloy wires

Titanium alloys such as Ti-6Al-4V and Ti-6Al-7Nb are widely used in the biomedical industry as structural implant materials. The strain-rate sensitivity of tensile strength is now assessed using a modified Kocks-Mecking formulation for hardening. The operative scale of the microstructural strengthening is designated by the coefficieint c b that can be determined from measurements of plastic strain, yield and ultimate strength. It is found that although that strength varies slightly with strain rate, the scale of the microstructure c b remains nearly constant for each material. Finally, a low c b -value of 14 is computed for Ti-6Al-4V that is consistent with the refined twophase microstructure needed to enhance both ductility beyond yielding and its ultimate strength.

36 MATERIALS SCIENCE↗